The present invention relates to lightweight threaded fasteners including a pin or bolt and a nut member and which are of a type particularly useful for aerospace applications and to dies for roll forming an optimized thread form on the pin or bolt.
In the aerospace industry a large number of fasteners such as threaded bolts and nuts, swage type fasteners such as lockbolts, rivets and the like are used in the manufacture of an aircraft. These fasteners can contribute a significant amount to the overall weight of the aircraft. Thus it is desirable to reduce the weight of the fasteners while still providing the necessary strength and other fastening characteristics as required.
In aerospace applications, one type of such threaded fasteners include bolts or pins which have a large diameter smooth, cylindrical shank portion extending from the bolt head and a smaller diameter portion at the opposite end which is threaded. A generally tapered and/or arcuate transition portion connects the larger diameter cylindrical shank portion to the smaller diameter threaded shank portion. The threads on the small diameter shank portion are routinely formed by rolling. The larger diameter smooth shank portion is adapted to be located in the openings of workpieces to be secured together and is intentionally smooth or uniformly cylindrical to maximize shear strength and for other purposes as may be required by the particular application. The threads on the smaller diameter shank portion are formed with crests which are of a diameter which is normally less than the large diameter shank portion to facilitate initial insertion through the workpiece openings. This is especially the case where the large diameter shank portion is dimensioned to provide an interference or very close tolerance clearance fit with the workpiece openings. The fasteners also include a nut member adapted to be threadably secured to the threaded bolt portion with a mating thread form.
It should be understood that the terms "pins" and "bolts" are used interchangeably in the aerospace fastener field for threaded fasteners and thus the use herein of one of the terms includes the other. Roll formed threads on the bolts are believed to have superior fatigue properties relative to bolts with machined or ground threads. Roll forming of bolt threads is a procedure generally well known in the threaded fastener art. A conventional thread rolling die has uniform thread forming ridges and grooves over most of its die face for making a uniform thread. The outer edges of the dies are, however, conventionally somewhat chamfered and/or radiused to remove sharp feather edges to improve wear resistance and increase die life. This chamfer and/or radius will result in the formation of the thread with a run-out section on the reduced diameter blank portion on which the rolled threads are formed. In such run-out sections the root depth and the crest height of the thread decrease to the end of the thread. The axial lengths of such run-out sections are typically measured from the center of the root of the adjacent fully formed thread of full root depth and full crest height to the center of the root at the end of the run-out.
Thus in the routine run-out section the thread is shallower than in the preceding threaded portion of the bolt where the thread is fully developed. Likewise, the crest of the thread, including the flanks which carry the tensile clamp load, in the run-out section will not be fully developed. At the same time, the nut member used with the bolt has a mating thread which when formed normally has a conventional run-in section at its leading end. In the run-in section the nut thread also is not fully formed with the minor diameter being increased resulting in the depth of the run-in thread being reduced. The form and extent of such bolt thread run-out and nut thread run-in and the extent of their effective engagement and/or overlap will have an affect on the overall length of the pin and nut as well as the amount of the overall tensile strength contributed by those sections.
In the aerospace industry since the contribution by the engagement, if any, of the run-out and run-in sections to the overall tensile strength of the fastener is routinely minimal at best, it has been an objective to limit the length of the run-out section of the bolt thread. A contributing factor to this objective is the fact that it has been common for the thread forms of the thread run-out and nut run-in to be such as to have interference when overlapping and hence such overlap is avoided requiring sufficient length of the bolt and nut to compensate as will be described. In this regard, however, it is still desirable to be able to have as much overlap as possible of the nut thread run-in with the bolt thread run-out without interference. At the same time it is also desirable to minimize the axial length of the large to small diameter transition portion. This is done in an effort to reduce the overall length and hence weight of the bolt and nut member. The combined axial length of the run-out section of the bolt thread and the adjacent section of the transition portion is referred to as the "run-out zone". In the past this has typically been in the range of around 2.0 P--where "P" is the thread pitch. In these cases the axial length of the run-out section on the bolt has typically been between around 1.0 P to around 1.5 P. At the same time, the conventional run-in section for the nut thread typically extends for a length of around 0.5 P.
As a practical, economic matter, it is common for a single fastener to be used to secure workpieces varying in total thickness with the maximum total thickness being referred to as the "grip" or "maximum grip" of the fastener and the minimum total thickness being the "minimum grip". Where the bolt of the fastener has a flush head, the "grip" or "maximum grip" is measured from the outer end of the flush head up to the end of the uniform smooth shank portion or up to a predetermined point within the transition portion coincident with the maximum grip for lightweight bolts or pins. For other head styles, such as a protruding head, the "grip" or "maximum grip" is measured from the beginning of the cylindrical, smooth shank portion up to the same noted locations described for the flush head. The plane corresponding to the maximum thickness of parts to be held by a bolt is sometimes also referred to as the "maximum grip" or "maximum grip plane". At the same time the plane corresponding to the minimum total thickness to be secured is sometimes referred to as the "minimum grip" or "minimum grip plane". In aerospace applications, in order to provide a reasonable range of grip capability and minimize excess weight, the difference between "maximum grip" and "minimum grip" for a single sized fastener is typically limited to 1/16 inch. For example, in aerospace applications where the total or maximum workpiece thickness for a single sized fastener is one quarter of an inch, the minimum grip for that fastener is one-sixteenth inch less than the maximum grip.
Thus where the total thickness of the workpieces is at maximum grip the transition portion will extend or protrude at least partially past the outer surface of the workpieces from the predetermined point. Where the total thickness of the workpieces is less than the maximum grip, the outer end of the smooth cylindrical shank portion and the entire transition portion will extend or protrude past the outer surface of the workpieces. Thus the threaded nuts used with such aerospace fasteners typically have an enlarged diameter counterbore at one end which is concentric with the threaded bore through the nut. The counterbore is sized to receive the protruding end of the smooth shank portion and transition portion without interference in all grip conditions from maximum grip to minimum grip. Consequently the axial length or depth of the counterbore in conventional threaded nuts is such that when fully tightened on the bolt, the threaded portion of the nut, including the nut run-in section, does not extend completely into the bolt run-out section. With such conventional fasteners if the nut thread including the run-in section were to extend into and completely overlap the run-out section there could be thread interference between the fully formed nut thread and the incompletely formed shallow root of larger diameter of the bolt run-out thread. This is, to some extent, because the length of the nut run-in of 0.5 P is less than the bolt run-out and the contour of the fully formed nut thread may not match or be otherwise compatible with the bolt thread run-out. Such interference could impede the nut from being properly tightened in securing the parts being fastened and/or create undesirable stress on the bolt and/or nut. As will be seen this is substantially improved with the fastener of the present invention. In this regard it should be noted that it is common for the fully formed threads of the nut and bolt to be made to have a slight clearance between their respective thread forms, i.e., roots and crests; thus this clearance could permit the nut thread including the run-in section and adjacent fully formed nut thread to extend somewhat farther into the bolt thread run-out section without interference.
The depth of the nut counterbore in a conventional aerospace fastener is the difference between the maximum grip and minimum grip of the bolt with about 2.0 P added to accommodate for run-out and other dimensional tolerance variations. This has been done with such conventional fasteners so that when the nut is used to secure workpieces of the minimum grip, the axially inner end or beginning of the nut thread run-in is at the axially outer end or beginning of the bolt thread run-out, which in the example noted, would be a distance of about 2.0 P from the maximum grip plane of the bolt which is proximate to the end of the cylindrical shank or maximum grip plane of the bolt. When the nut is used to secure workpieces having the maximum grip, the thread at the inner end or beginning of the nut run-in stops at about the length of the "difference" between the maximum and minimum grips plus around 2.0 P from the outer end or beginning of the bolt thread run-out.
Thus it can be seen that, if the length of the bolt thread run-out zone is reduced, the length of the counterbore on the nut could be correspondingly reduced. This would permit a reduction in the overall length of the nut and could also result in a corresponding shortening of the bolt. Because of the large number of fasteners used in an aircraft, the total weight savings per aircraft by reducing the length of the nut and bolt, separately or together, by even slight amounts could be significant.
One approach to reducing the length of the bolt thread run-out section has been to form a bolt blank with a relief groove in the section between the transition portion from the large diameter, smooth shank portion and the smaller diameter portion on which the threads are to be rolled. The groove is approximately formed to the minor diameter of the thread to be produced. Here the groove, which is cut prior to rolling the thread, is then rolled to increase its strength by eliminating tool marks induced by machining. The thread is then rolled, with what would be the thread run-out of a formed thread extending into the groove. This results in an actual thread run-out section which is shorter than that of prior conventional fasteners with a rolled thread. This type of bolt and thread form is generally as shown in U.S. Pat. No. 4,485,510 issued Dec. 4, 1984 and U.S. Pat. No. 4,957,401 issued Sep. 18, 1980 to Hatter for "Threaded Fastener Having Minimized Length And Weight And Method To Make It". It can be readily understood, however, that these extra steps in cutting and rolling the groove could result in increased expense over a bolt in which the threads are simply roll formed without such a relief groove. With such latter bolts it is understood that from the maximum grip plane the relief groove is within 1.0 P but that a full thread with a full crest is within 1.5 P.
In contrast another, subsequently developed, form of bolt for a lightweight fastener has a rolled thread with an effective run-out zone of only about 1.0 P where both the root and crest are fully formed. Here a special rolling die is used which, instead of tapering the depth of the root of the thread in the run-out section, forms a thread of full depth to within 1.0 P of the maximum grip plane. The run-out section is formed generally in a section of the transition portion which connects the large diameter shank portion to the smaller diameter shank portion on which the rolled thread is fully formed and is of a maximum length of one-quarter, and usually less, of the circumference at the pitch diameter. The root of the thread in the transition portion is generally of full depth except at the end of the run-out where it increases abruptly to the larger diameter of the transition portion at that location. This has permitted reduction of the total length of the nut and bolt by around 1.0 P from a conventional nonlightweight fastener. This type of bolt is generally as shown and described in U.S. Pat. No. 4,785,537 issued Apr. 5, 1988 to Rath for "Thread Rolling And Fastener". Here the run-in of the conventional nut thread does not match the minimal, abrupt run-out of the bolt thread at the large diameter shank portion. In one commercial form of that fastener the run-out is formed on the smaller diameter shank portion and sharply increases at the end but somewhat less abruptly. However, such abrupt run-outs can be functionally accommodated by the nut counterbore and normal nut thread clearance whereby the same nut with the conventional nut thread run-in still can be used here as well as on the previously noted bolt with the relief groove whereby bolts of both constructions can be used interchangeably with a common nut member.
In addition to the preceding, there are a number of other patents directed to bolts with a thread run-out of various forms and lengths between the larger diameter smooth, cylindrical shank portion and the fully developed threaded portion. For example see U.S. Pat. No. 4,842,466 issued Jun. 27, 1989 to Wheeler et al for "Lightweight Fastener" in which the bolt is constructed with a thread run-out zone of between 1.6 P to 2.0 P which falls generally within a triangular envelope; the nut thread is specially formed to have a matching conical run-in. See also U.S. Pat. No. 4,915,559 issued Apr. 10, 1990 to Wheeler et al for "Lightweight Fastener" in which the bolt thread run-out zone is between 1.58 P to 1.8 P and follows a concave curve with the nut thread being specially formed to have a matching convexly curved run-in. In addition see U.S. Pat. No. 5,039,265 issued Aug. 13, 1991 to Rath et al for "Lightweight Fastener" in which the bolt thread run-out zone is between 1.4 P to 2.3 P and follows an S-shaped curve with a concave portion and with the nut thread crests being truncated to match. With these latter fasteners, it would appear that a nut with a conventional thread run-in would not be suitable and that different nut members with specially adapted run-ins would be required. Likewise, it would appear that these different nut members would not be readily useable with the bolts of the cut groove or abrupt run-out type previously described and hence, where a nut member of one design is to be used, the bolts with the cut groove or abrupt run-out would not be used interchangeably with those of the '466, '559 and '265 patents as described above.
U.S. Pat. No. 5,788,441 is directed to a threaded bolt with a rolled thread in which the thread in the run-out section extends helically with the depth of the thread decreasing from full thread depth to no thread depth within one-third of the circumference of the rolled thread at its pitch diameter. Here, however and as with the '265 patent noted above, the thread run-out is formed on a separate, transition shank portion which differs in diameter from both the large diameter smooth shank portion at one end and the small diameter shank portion with fully formed thread at the other end. The threaded bolt of the '441 patent would appear to be adapted for use with the same nut member as the bolts of the cut groove or abrupt run-out.
In the present invention a threaded fastener is provided in which the thread is roll formed on a reduced, uniform diameter shank portion of a bolt blank. At the same time a unique thread run-out section is formed solely on the same reduced diameter shank portion and is of a minimal total length extending axially from the beginning of the thread run-out section for no greater than around 1/4 P. In this regard the thread run-out section extends circumferentially for no more than between around 1/3 to around 1/2 of the circumference of one thread pitch. Here the root depth and crest height decrease gradually to a relatively smooth, arcuate termination at the end of the run-out section. The full thread and the run-out thread, however, are all, advantageously, formed on the reduced, uniform diameter shank portion. At the same time the run-out section is of a unique construction having an end wall extending parallely and located closely adjacent to the generally tapered or arcuate transition portion which connects the large diameter, smooth pin shank portion to the reduced diameter threaded shank portion. In addition the present invention permits the axial length of the transition portion to be minimized. With the fastener of the present invention constructed as noted above, the run-out zone, as previously defined, can be minimized to have a fully formed thread at a maximum axial distance from the maximum grip plane of no greater than 1.0 P. The minimal run-out zone formed by the limited run-out section and the included section of the arcuate, tapered transition portion along with the overall reduced axial length of the transition portion provide a bolt of minimal size and weight. While the run-out section is quite limited, it readily permits the use of a nut with a conventional thread run-in and with a counterbore of reduced depth. This results in a fastener assembly of a bolt and nut of minimized overall axial length and weight. This also permits the interchangeable use of the bolt of the fastener of the present invention with the previously described bolts with the relief groove, bolts with the abrupt run-out and those with a tapered run-out on a separate transition portion through a common nut member with the conventional run-in.
In the present invention a uniquely constructed rolling die is utilized to roll form the bolt or pin thread with this desired run-out. In the thread rolling die of the present invention, ridge segments at the upper or outer edges of the dies are formed on a plurality of stepped beveled surfaces and the upper flanks are tapered at one end to provide a gradual run-out avoiding the formation of a run-out generally as an abrupt end wall. At the same time the upper ridge segments on the thread-rolling dies of this invention do not come to a feather edge as they run out on the edge of the die where the inner end of the thread is formed. Rather the run-out sections at the upper ridge segments extend for a relatively short distance with the inner flank being parallel to the confronting flank of the opposite die ridge. Also, by forming the run-out on the same reduced diameter shank portion as the full thread, die life is not impaired and is believed to be improved. In this regard it is common in prior constructions, where the run-out section extends into a portion of the blank of different diameter than that where the full thread is formed, for laps, folds and other undesirable features to be generated as a consequence. Such undesirable features are minimized in the present invention where the run-out section is formed totally on the same small diameter portion as the full thread.
Thus it is an object of the present invention to provide a lightweight threaded fastener including a bolt which has a threaded shank portion with a unique, gradual run-out of minimal length and a fully formed thread root and crest within one pitch, 1.0 P, of the maximum grip plane.
It is another object of the present invention to provide such a lightweight fastener in which the run-out section of the bolt thread extends gradually for an axial length of no greater than around 1/4 P and circumferentially for no greater than from around 1/3 to 1/2 of the circumference at the pitch diameter.
It is still another object of the present invention to provide a lightweight threaded fastener in which the run-out section of the thread of the bolt is of a unique form of limited length and can be used with a nut having a mating thread with a conventional run-in of about 0.5 P.
It is another object of the present invention to provide a lightweight threaded fastener including a bolt in which the thread run-out is formed totally on the same small diameter shank portion as the full threads whereby laps, folds and other undesirable features are minimized.
It is an object of the present invention to provide a lightweight threaded fastener in which a bolt has an enlarged diameter smooth shank portion connected to a small diameter shank portion by a smooth transition portion of minimal length and with a thread, including a unique, short run-out section, formed substantially solely on the reduced diameter shank portion.
It is an object of the present invention to provide a lightweight threaded fastener in which a bolt has an enlarged diameter shank portion connected to a smaller diameter shank portion by a smooth transition portion of minimal length and with a thread, including a unique short run-out section, formed substantially solely on the reduced diameter shank portion and extending for an axial length of no greater than around 1/4 P and located closely adjacent to the transition portion to provide a run-out zone of minimal axial length of from around 2/3 P to no greater than 1.0 P.
It is an object of the present invention to provide a lightweight threaded fastener in which a bolt has an enlarged diameter smooth shank portion connected to a small diameter shank portion by a smooth transition portion of minimal length and with a thread, including a unique, short run-out section, formed substantially solely on the reduced diameter shank portion and extending for an axial length of no greater than around 1/4 P and circumferentially for a distance of between around 1/3 to around 0.5 of the circumference at the pitch diameter and which can be used with a nut having a conventional run-in of about 0.5 P of a contour generally matching and/or compatible with the bolt run-out.
It is another object of the present invention to provide a lightweight threaded fastener in which a bolt has an enlarged diameter smooth shank portion connected to a small diameter shank portion by a smooth transition portion of minimal length and with a thread, including a unique, short run-out section, formed solely on the reduced diameter shank portion and with the run-out section having an end wall extending parallely and being closely adjacent to the transition portion whereby the axial length of the run-out zone is minimized.
It is still another object of the present invention to provide a rolling die of a unique construction to roll form the thread on a shank portion of a bolt having the unique gradual run-out of minimal length as noted and with the full thread and thread run-out formed on a common shank portion which is of a substantially uniform diameter.
Other objects, features, and advantages of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings, in which: